In particularly large caliber ordnance such as artillery guns, only two different types of mechanisms are fundamentally conceivable. The first and most widely used but at the same time also most expensive and difficult to manufacture is the so-called screw mechanism in which the loading aperture of the gun barrel is closed by means of a special impact base with associated obturation pad, a so-called Crossly pad, which in turn is supported by a chamber screw functioning as an abutment. This latter displays outer alternatingly smooth and threaded sectors which are inserted into a correspondingly adapted and correspondingly sectorwise threaded locking portion in the breech piece of the gun, where the chamber screw is then locked by a turning movement which causes the different threads to intermesh. The major advantage inherent in this type of mechanism is that it is so gastight that it can be employed for caseless ammunition, for example when the gun is loaded with a shell and one or more separate propellant charges, so-called bag charges, or when use is made of combustible cases, and thus provides no sealing aid on firing as is provided by a cartridge case expanded by the gas pressure.
The second basic type of mechanism is the so-called sliding breech mechanism in which use is made of a breech mechanism displaceable transversely of the loading aperture of the gun barrel in a groove adapted therefore in the breech piece of the gun fixedly connected to the gun barrel, for rearward closing and sealing of the gun barrel. Even if the breech mechanism may be provided with such a path of motion that it is urged against the rear portion of the gun barrel when it is displaced from its open loading position to its fully closed firing position, this type of mechanism will not in itself be sufficiently gastight for use with other than cased ammunition. On the other hand, it functions very well in connection with such cased ammunition, in which the cartridge cases may be utilized as a seal against the inner gas pressure.
However, a considerable amount of development work has been devoted to producing "tight" breech mechanisms. In one of these, use is made of an annular resilient sealing plate of metal as a seal between the gun barrel and the breech mechanism. Such sealing plate has an angular cross-section and is turned with its one flank to face the outer edge of an annular groove disposed about the inside of the rear portion of the gun barrel, while the other free flank of the sealing plate is turned to face the breech mechanism. The sealing function is based on the concept that the gas pressure within the gun barrel will urge the free flank of the sealing plate against the block mechanism and seal thereagainst. The design as such may be rendered sufficiently tight for employment together with caseless ammunition, but at the same time it is prone to damage, not least to the resilient sealing flank of the sealing plate and consequently there has been general reluctance to introduce this type of design, since such damage to the sealing plate may cause a considerable blow-back, which would of course constitute a serious risk to the gun crew.
Patent specification EP-A1-0014559 further describes a two-part breech mechanism (previously unknown to us) comprising a first locking device displaceable transversely of the longitudinal direction of the gun barrel and a sealing device interconnected therewith which, in turn, is displaceable both transversely of the gun barrel and, in the closing position, partly into the rear opening thereof where it is blocked by the locking device. The configuration of that part of the sealing device which is inserted in the rear region of the gun barrel is described in that specification as a mushroom provided with a stem seal disposed about the neck of the mushroom. The problem which may be directly foreseen with this design is that the stresses on parts of the stem seal must be extremely high, since parts thereof, even if the greater part is protected by the mushroom hat, are disposed directly in the line of fire from the propellant charge combustion.